Structural Modification of Silk Fibroin via Acylation for Biobased Plastics

Silk fibroin (SF) exhibits excellent mechanical properties closely associated with its crystalline structure. Silk resin is an important plastic material for environmental applications. In this study, we investigated how different carbon chains affect the crystalline structure and physical properties of bulk SF resins. SF was modified with five carboxylic anhydrides of varying chain lengths and steric hindrance and was fabricated into bulk resins via thermal compression molding. Nuclear magnetic resonance spectroscopy revealed that long or bulky carbon chains altered the secondary structure of SF. X-ray diffraction demonstrated the expansion of the spacing between β-sheets and changes in the stacked sheet structure depending on the carbon chain structure, as supported by differential scanning calorimetry through altered thermal stability. Mechanical testing showed differences in bending strength and modulus, with SF modified by a tert-butyl-containing acyl group maintaining high strength despite reduced crystallinity. Overall, carbon chain modification effectively tunes the crystallinity and macroscopic properties of bulk SF resins, enabling the development of biobased plastics from silk resources.